A new broadband electromagnetic (EM) sensor, called GEM-5, for shallow earth investigation<br>including detection and discrimination of UXO has been developed. The sensor consists of a central<br>transmitter coil and two coaxial receiver coils located at the same vertical distance above and below the<br>transmitter coil. The difference in output between the receiver coils would be zero in free space. This<br>particular configuration provides a high degree of primary field cancellation, reduction of power-line or<br>other environmental noise, and immunity from noise induced by sensor motion in the earth’s magnetic<br>field.<br>We have first studied the detection capability of the sensor. The EM responses are numerically<br>simulated using a sphere model for UXO-like objects, and a half-space for geology. Algorithms to invert<br>the EM data into apparent susceptibility and conductivity show that the sensor can detect small metal<br>targets. Transforming the EM data into the apparent conductivity can enhance the anomalies from metal<br>targets and reduce geological noise. Our field test indicates that the gradiometer measurements yield a<br>lower static and motion noise over the critical low frequencies than that of existing sensors, and detect<br>all targets at a seeded UXO test site.


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